IST hands down and plan so you have lots of down time in the airport so you can enjoy one of the most fabulous lounges anywhere. Just beware when you are booking with TK that there are TWO international airports in Istanbul and you really do not want to travel from one to the other to continue your journey. Been there; done that only because the flight times out of IST were brutal - 6 am departures, instead of 2 pm. We were going from SFO to Kiev by way of IST. Pray that the airport still allows previewed passengers in the wiki line because the public line (mad house) is brutal.